For example, the SEM described in Japanese Unexamined Patent Publication No. 2007-42513 (Patent Literature 1) is configured to scan a sample by irradiating the sample with a narrowly-focused electron beam as probe and moving the position of irradiation with the electron beam, and to detect electrons (secondary electrons or reflected electrons) generated at each position of the sample in accordance with the irradiation with the electron beam, by a detector. The SEM can perform a surface analysis of the sample by such electron detection. SEM has the feature of larger magnifying powers than optical microscopes and is indispensable as apparatus for visualizing nanoscale microstructures, e.g., for inspection of semiconductor mask patterns.
When a sample is irradiated with the electron beam, the sample generates secondary electrons or reflected electrons. The secondary electrons are electrons generated from near the surface of the sample. An image obtained by detection of secondary electrons (secondary electron image) reflects microscopic unevenness of the sample. The reflected electrons are electrons recoiling upon collision with atoms constituting the sample. Since the number of reflected electrons depends on a composition of the sample (an average atomic number, crystallographic orientation, or the like), an image obtained by detection of reflected electrons (reflected electron image) reflects a composition distribution of the sample. The surface analysis of the sample can be implemented based on the secondary electron image or the reflected electron image.
Since the probe used in SEM is the electron beam, it is not difficult to increase or decrease an electron dose of the electron beam irradiating the surface of the sample. On the other hand, in measuring an insulator sample or the like, a charging phenomenon occurs, resulting in degradation of image quality. For this reason, SEM has been used heretofore while adopting countermeasures such as limitation of the incident electron amount or a quenching gas for prevention of charging.